Direct Coupling of Plasmonic andPhotonic Nanowires for HybridNanophotonic Components and CircuitsXin Guo,† Min Qiu,‡ Jiming Bao,§ Benjamin J. Wiley,| Qing Yang,† Xining Zhang,†Yaoguang Ma,† Huakang Yu,† and Limin Tong*,†

State Key Laboratory of Modern Optical Instrumentation, Department of OpticalEngineering, Zhejiang UniVersity, Hangzhou 310027, China, Lab of Photonics andMicrowaVe Engineering, School of Information and Communication Technology, RoyalInstitute of Technology, Electrum 229, 16440 Kista, Sweden, Department of Electricaland Computer Engineering, UniVersity of Houston, Houston, Texas 77204, andDepartment of Chemistry, Duke UniVersity, Durham, North Carolina 27708

Received September 1, 2009; Revised Manuscript Received October 15, 2009

ABSTRACT

We report direct coupling of plasmonic and photonic nanowires using ultracompact near-field interaction. Photon-plasmon coupling efficiencyup to 80% with coupling length down to the 200 nm level is achieved between individual Ag and ZnO nanowires. Hybrid nanophotoniccomponents, including polarization splitters, Mach-Zehnder interferometers, and microring cavities, are fabricated out of coupled Ag andZnO nanowires. These components offer relatively low loss with subwavelength confinement; a hybrid nanowire microcavity exhibits a Q-factorof 520.

Nanowires are intriguing materials for fundamental studiesof light-matter interactions at the nanoscale, as well as forfabrication of nanophotonic components and circuits.1-3

Photonic nanowires such as semiconductor nanowires,4 glassnanofibers,5 and silicon-on-insulator (SOI) waveguides6

exhibit low optical loss, but optical confinement in thesestructures is limited by the diffraction limit (on the order ofλ/n, with refractive index n typically less than 5). By couplinglight to free electron oscillations in the metal, plasmonicnanostructures provide the possibility to break the diffractionlimit and confine light to scales less than λ/10,7,8 as well asto carry optical and electrical signals in the same optoelec-tronic circuitry.9 However, plasmonic nanostructures usuallysuffer from high losses when light is confined to subwave-length scales, preventing the realization of “all-metal”nanophotonic circuitry. A number of strategies,10 such aschannel plasmon polaritons,11 plasmonic waveguiding withgain medium,12,13 and hybrid approaches combining plas-monic with dielectric waveguiding14-17 have been suggestedto circumvent this problem, but simultaneous realization of

subwavelength confinement and low propagation loss re-mains a challenge. By substituting low-loss photonicwaveguides for metallic counterparts where ultratight con-finement is unnecessary, it is possible to fabricate nanopho-tonic components with relatively low losses, in which thekey issue is efficient coupling between the two types ofwaveguides. In the past years, a variety of approaches forcoupling plasmonic and photonic waveguides have beenproposed or realized.14-19 Compared with the wavelength ofthe light, the overall size of these structures remains large.Here we demonstrate, for the first time, direct coupling ofplasmonic and photonic nanowires via sub-wavelength-scalenear-field interaction. Using efficiently coupled plasmonicand photonic nanowires, we further demonstrate the pos-sibility of realizing hybrid nanophotonic components/circuitswith subwavelength confinement and reduced loss.

The photonic nanowires used here are silica nanofibers(taper drawn from single-mode fibers5) and ZnO nanowires(synthesized via a chemical vapor transport process20), whichexhibit low loss (<1 dB/mm) for optical waveguiding.4,5

Chemically synthesized Ag nanowires with smooth surfacesare used as plasmon waveguides.21 The Ag nanowires werefirst synthesized and washed using the same methods as havebeen reported elsewhere,21,22 suspended in an aqueoussolution, and then deposited on a MgF2 substrate. When thetwo types of nanowires are placed close enough, optical near

* To whom correspondence should be addressed, phytong@zju.edu.cn.† State Key Laboratory of Modern Optical Instrumentation, Department

of Optical Engineering, Zhejiang University.‡ Lab of Photonics and Microwave Engineering, School of Information

and Communication Technology, Royal Institute of Technology.§ Department of Electrical and Computer Engineering, University of

Houston.| Department of Chemistry, Duke University.

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10.1021/nl902860d CCC: $ XXXX American Chemical Society

field in a sub-wavelength-diameter photonic nanowire andplasmonic near field in a plasmonic nanowire may stronglyoverlap, resulting in highly efficient photon-plasmon con-version in the coupling area.

Experimentally, light used for excitation was first lens-coupled into a standard fiber (SMF 28, Corning), and thensqueezed into the nanofiber through a fiber taper, yieldingquasi-circular-polarization guided modes (HE11 modes) withhigh fractional evanescent fields.23 The waveguiding nanofi-ber was then brought into contact with ZnO or Ag nanowiresby micromanipulation (see Supporting Information). Asschematically illustrated in Figure 1a, light from a nanofibercan excite surface plasmons in the Ag nanowire directly (top)or travel through an intervening ZnO nanowire beforelaunching plasmons in the Ag nanowire (middle). Con-versely, plasmons in the Ag nanowire can convert back intolight guided by the ZnO nanowire (bottom). All the nanow-ires are supported on a nondissipative substrate for low-lossoperation. Figure 1b shows an optical micrograph of 650nm wavelength light from a nanofiber coupled into a 320nm diameter Ag nanowire supported on a MgF2 substrate(n ) 1.38). With an overlap between the nanofiber and thenanowire of less than 1 µm, light scattered from the outputof the 15.7 µm length Ag nanowire, indicating effectiveexcitation of propagating plasmons in the Ag nanowire. InFigure 1c, the light from the nanofiber coupled into a 340nm diameter 19.3 µm length ZnO nanowire, and theneffectively excited plasmons in the Ag nanowire, with anoverlap of about 2 µm (see Figure 1d). Figure 1e illustratescoupling of plasmons from the excited Ag nanowire backinto guided light in the ZnO nanowire.

Polarization of nanowire outputs in Figure 1e was inves-tigated with a linear polaroid. When the polarization isparallel to the nanowires (P|), the output from the Agnanowire is maximized and the output from the right end ofthe ZnO nanowire is minimized (see Figure 1f). Conversely,

when the polarization is perpendicular to the nanowires (P⊥),the output from the ZnO nanowire is maximized and theoutput from the Ag nanowire is minimized (see Figure 1g).This can be explained by the different polarizations of guidedmodes in the two types of nanowires: the Ag nanowire onlyaccepts P|-polarized modes as guiding modes, while the ZnOnanowire mainly supports P⊥-polarized modes. When lightis coupled from the Ag to the ZnO nanowire, the strong near-field interaction enables the efficient transfer: the output ofP|-polarized plasmonic modes in the Ag nanowire, whichotherwise radiates into free space with polarization perpen-dicular to the propagation direction at far field, was efficientlypicked up by the ZnO nanowire at near field due to the strongmode overlap of the two nanowires and evolved into guidingmodes of ZnO nanowire at far field. The high extinction ratioof the nanowire outputs for orthogonal polarizations (over12 dB) provides an opportunity for nanoscale polarizationcontrol.

A hybrid coupler can be readily assembled using a ZnOnanowire and a Ag nanowire, as illustrated in Figure 2a. Sucha coupler was experimentally realized with a 270 nmdiameter ZnO nanowire and a 240 nm diameter Ag nanowiresupported on a MgF2 substrate, with a coupling length assmall as 220 nm (see Figure 2b). The fractional outputs fromthe Ag nanowire, obtained as the ratio of output intensity ofthe Ag nanowire to the total output of the both nanowires(see Figure 2c-e), are 4%, 8%, and 64% for 488, 532, and650 nm light, respectively (see Supporting Information). Thehigher fractional output of propagating plasmons at longerwavelengths is also verified in Figure 2f, in which a white-light excitation results in red light output from the Agnanowire. After the guiding losses of the Ag nanowire (about0.43 dB/µm,19,24 or 3 dB for the 7.2 µm length) and the ZnOnanowire (lower than 0.001 dB/µm and therefore negligible)were deducted, the calibrated coupling efficiency from theZnO nanowire into the Ag nanowire at 650 nm wavelength

Figure 1. Direct coupling of plasmonic and photonic nanowires. (a) Schematic illustration of light coupling between silica nanofibers(located at distal end of the fiber taper), ZnO nanowires, and Ag nanowires. Light from the nanofiber can excite plasmons in the Agnanowire directly (top) or via an intervening ZnO nanowire (middle). Plasmons in the Ag nanowire can convert back to light in the ZnOnanowire (bottom). (b, c, e) Experimental observations corresponding to the three different coupling schemes illustrated in (a). (d) SEMimage of the coupler shown in (c). The diameter of the ZnO nanowire is 340 nm and the diameter of the Ag nanowire is 320 nm. (f, g)Microscope images taken with the polarization along and perpendicular to the nanowires shown in (e), respectively. Scale bar in (b) appliesto (c-g).

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(see Figure 2e) is about 82%. Very recently, numericalsimulation predicted a coupling efficiency up to 95% whencoupling light from a dielectric nanofiber to a metallicnanowire using a butt-coupling scheme,25 indicating higherefficiency is also possible to realize in the side-by-sidecoupling scheme reported here.

Compared with many other coupling schemes for plas-monic nanowire excitation,19,22,24,26 the nanowire-to-nanowirecoupling approach offers a much higher efficiency. The highefficiency obtained here benefits from the tightly confinedsurface modes (evanescent fields) in the ZnO nanowirewhich, when scattered by the Ag nanowire, create consider-able overlap between the incident optical radiation and thepropagating plasmon mode. With an orthogonal polarization,output from the Ag nanowire vanishes (see Figure 2g),demonstrating the possibility of operating the hybrid coupleras an ultracompact polarization splitter. Compared with anyprevious polarization splitter,16,27 the hybrid nanowire splitteroffers a coupling length 10 times smaller. Figure 2h shows650 nm light guided along a single 320 nm diameter ZnOnanowire was converted into propagating plasmons inmultiple Ag nanowires (insets), representing an ultracompactapproach to a recently reported idea for hybrid integrationof photonic and plasmonic waveguides.19

By coupling both ends of a Ag nanowire to a ZnOnanowire (see Figure 3a), we assembled a hybrid Mach-Zehnder interferometer (MZI). Figure 3b shows an opticalmicroscope image of the MZI assembled using a 330 nmdiameter ZnO nanowire and a 120 nm diameter Ag nanowireon a MgF2 crystal, with white light launched in and pickedup by fiber tapers (see Supporting Information). The experi-

mentally measured transmission spectrum of the MZI is givenin Figure 3c, clearly showing interference fringes with a freespace range (FSR) of about 2.75 nm at a wavelength of 710nm. Theoretically, FSR of the hybrid Mach-Zehnder inter-ferometer is given by

where λ is the wavelength, LZnO (89 µm) and LAg (6.5 µm)are the path lengths of the ZnO and Ag nanowires, respec-tively.28 ng,ZnO and ng,Ag are group indices of ZnO and Agnanowires obtained by ng ) n - λdn/dλ (n is the index ofrefraction at wavelength λ). At λ ) 710 nm, for ZnO n )1.97 and dn/dλ ) -0.18 µm-1, for Ag n ) n1 + ik ) 0.15+ 4.74i and dn1/dλ ) 0.044 µm-1,29 the calculated FSR isabout 2.71 nm, which is in good agreement with theexperimental measurement.

By use of the complementary ZnO nanowire for opticalconnection and redirection, the use of the Ag nanowire isminimized, resulting in relatively low insertion loss for theMZI. Due to the favorable waveguiding properties ofplasmonic nanowaveguides8,30 including tight confinement,field enhancement, and ultrafast nonlinear response, thehybrid nanowire MZI demonstrated here offers great potentialfor achieving ultracompact functional components anddevices ranging from ultrafast optical modulators30 to opticalsensors.31

Connecting a Ag nanowire with a ZnO nanowire in aclosed loop yields a hybrid plasmonic microring cavity. Asschematically illustrated in Figure 4a, two additional nanofi-

Figure 2. Hybrid couplers fabricated out of coupled Ag and ZnO nanowires. (a) Schematic of a hybrid coupler. (b) SEM image of a hybridcoupler assembled from a 270 nm diameter ZnO nanowire and a 240 nm diameter Ag nanowire. (c-f) Optical micrographs of the hybridcoupler taken with the polarization parallel to the Ag nanowire when monochromatic lasers of wavelength 488, 532, and 650 nm and whitelight from a halogen lamp are used as light sources. (g) Optical micrograph of the coupler with the 650 nm source when the polarizationis perpendicular to the Ag nanowire. Scale bar in (c) is applicable for (d-g). (h) Excitation of multiple Ag nanowires integrated with asingle ZnO nanowire. Ag nanowires from left to right are 90, 165, 180, 160, and 115 nm in diameter, with coupling angles ranging from5° to 68°. Insets, close-up SEM micrographs of typical coupling structures.

FSR ≈ λ2/(LZnOng,ZnO - LAgng,Ag)

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bers and ZnO nanowires were used for optical launching andsignal collection with an add-drop technique. Figure 4bshows a SEM micrograph of the hybrid structure assembledwith a 400 nm diameter ZnO nanowire and a 265 nmdiameter Ag nanowire. The hybrid cavity was characterizedby launching a white light at the input port (upper left) andcollecting signals from the drop port in the upper right (seeinset of Figure 4c). Spectral response given in Figure 4cshows the resonance feature of the cavity, with measuredFSR of about 3.8 nm. The FSR of the hybrid microring cavitycan be estimated by

where λ is the resonant wavelength.28 LZnO ) 99.5 µm, LAg

) 16.5 µm. At λ ) 890 nm, for ZnO n ) 1.95 and dn/dλ )-0.08 µm-1, for Ag n ) n1 + ik ) 0.16 + 5.95i and dn1/dλ

) 0.41 µm-1.29 The calculated FSR is about 4.01 nm,corresponding well to the experimental measurement. TheQ-factor of the hybrid cavity, obtained from the full widthat half-maximum (fwhm) of the resonant peak around 890nm, is about 520. Previously reported Ag nanowire cavitiesof the Fabry-Perot type offer Q-factors less than 100; mostlight is lost due to the low percentage of light reflected fromthe ends (about 25%).24 The hybrid structure shown herehas similar resistive losses in the Ag nanowire but has greatlyimproved light recirculation because of the high couplingefficiency (up to 80%) between the ZnO and Ag nanowiresand low loss of the ZnO nanowire. The nanowires could bebent to a much smaller radius (on the order of 1 µm),4

offering possibilities for making hybrid cavities with muchsmaller sizes. In addition, the use of high-gain semiconductornanowires also provides an opportunity for compensatinghigh propagation losses of plasmonic nanowires withoutsacrificing the tight confinement, enabling fabrication ofultracompact components with low signal attenuation.

The seamless integration of photonic and plasmonicnanowires offers a way to overcome the limitations of

Figure 3. Hybrid Mach-Zehnder interferometer (MZI). (a) Sche-matic of a hybrid MZI. (b) Optical microscope image of the MZIassembled using a 330 nm diameter ZnO nanowire and a 120 nmdiameter Ag nanowire on a MgF2 crystal, with a white lightexcitation source. Inset, a SEM image of the coupling area. Scalebar, 2 µm. (c) Transmission spectrum measured with the white lightsource.

FSR ≈ λ2/(LZnOng,ZnO + LAgng,Ag)

Figure 4. Hybrid microring cavity. (a) Schematic of a hybridmicroring cavity. (b) SEM image of a cavity assembled using a265 nm diameter Ag nanowire and a 400 nm diameter ZnOnanowire in a closed loop. (c) Drop signal of the hybrid cavitycharacterized with a white light. Inset, dark field optical microscopeimage of the cavity under investigation.

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entirely photonic or plasmonic systems and enables thefabrication of nanophotonic components and devices withunprecedented performances. The silica nanofiber input/output scheme utilized here provides a method for connectingnanophotonic/plasmonic circuits with outer fiber communica-tion systems. Also, hybrid nanophotonic/plasmonic systemswill enable the fabrication of extremely high densities ofoptoelectronic devices that exhibit relatively low loss,32 aswell lead to useful tools for fundamental research innanoscale photonics, plasmonics, and quantum optics.33,34

Acknowledgment. This work was supported by theNational Basic Research Program of China (No. 2007CB-307003), the National Natural Science Foundation of China(Nos. 60728309 and 10974178), the Swedish Foundation forStrategic Research (SSF), and the Swedish Research Council(VR). Fruitful discussions and experimental assistance fromZ. Ma and W. Dai are also gratefully appreciated.

Supporting Information Available: Optical microscopeimage of a typical fiber taper, assembly processes of hybridcomponents with ZnO and Ag nanowires, and estimation offractional outputs and coupling efficiency of the ZnO andAg nanowires. This material is available free of charge viathe Internet at http://pubs.acs.org.

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